Color display systems have employed any of a variety of means for supplying the color component of an image. This includes systems that simply project light through a color transparency onto a projection surface. It includes cathode ray tubes (CRTs) with an array of areas of red, green, and blue phosphor that can be activated by electron beams as are commonly used in many color televisions and computer monitors. Of course, almost any color can be created by selecting the appropriate relative intensities of these three primary colors. Many front and rear projection display systems have three separate CRTs, one for each of red, green, and blue. In another type of projection system, the image is formed by reflecting light off of a spatial light modulator (SLM) that can be externally controlled to effectively turn on or off each of the small picture elements (pixels) of the SLM. Color is provided in such a system either by sequentially providing red, green, and blue light to the SLM or by utilizing three separate SLMs, one for each of red, green, and blue, and superimposing the image from the three SLMs to create the full color image. One method of sequentially providing red, green, and blue light to the SLM is by placing a rotating wheel having three segments disposed around its periphery, each being a filter to provide one of the three primary colors, in front of a white light source to selectively filter the white light into the three primary colors. This is a type of field sequential color system.
Field sequential color display systems using color wheels date back to the earliest days of the development of color television in the United States. At one time in the early 1950s the Federal Communications Commission issued a report (Public Notice 50-124) formally adopting a field sequential display system developed by the Columbia Broadcasting System as the US standard for color broadcasting. While this decision was eventually rescinded, field sequential color display devices based on the color wheel have continued to appear in the marketplace. Most recently a number of display manufacturers have offered front projectors using field sequential color generated by a color wheel to generate images for display to audiences.
Generation of images for display to groups of people requires great attention to detail in the design of the generating equipment. Typical goals include achieving high contrast, brightness, sufficient color saturation and balance, minimal artifacts, and clear, crisp images. Achieving suitable brightness while maintaining color balance has been a particularly difficult goal and is an objective of the present invention.
For example, U.S. Pat. Nos. 5,668,572 and 5,680,180 teach a color wheel with three color filter segments, where the relative size of each of the three color filters can be selectively optimized to achieve a desired color output. While this may provide the desired color, it does not address the brightness issue that is inherent to many projection display applications.
As is known, there are different aspects of color balance to consider in displays. One is to achieve the correct relative balance between the primary colors of red, green, and blue so that multicolor images assembled from these primary colors appear to be correct as well. In other words, this type of color balancing is based on human perception. The color balance of a system is often expressed as a triangle drawn on a CIE 1931 Standard Observer chart in the form of a coordinate pair for each color. A different perspective to color balancing is to achieve the correct color temperature as measured by instruments and expressed in degrees Kelvin. This is most often described as the white point color temperature for a display and it provides an important reference to users on the acceptability of the display for various applications and to different cultural groups.
Background to this invention is also found in U.S. Pat. No. 5,233,385, where brightness enhancement for sequential and spatial display systems is taught by adding to the system a non-filtered, transparent segment in addition to the three color filter segments that each pass a single color of light while reflecting or absorbing the remaining light, such as is shown in FIG. 1. While this is understood to increase the brightness of the image, it does not address controlling the nature of the light that passes through the transparent segment and that eventually appears on the projection surface.
For at least two reasons, providing a transparent section in a color wheel may be less than desirable. First, the illuminating light source may produce light that is not truly achromatic, meaning it does not have equal perceived intensity across the color spectrum of visible light. For example, some light sources have a spike in the yellow wavelength region. Second, since as discussed above the “whiteness” perceived by a human may be different than that measured by color temperature, it may be desirable to provide light that is not actually white light as measured but will be perceived as white by a human.
None of the approaches taught in the prior art address the issue of the color balancing of a brightness-enhanced image where the brightness enhancement is the result of addition of a transparent segment to the color wheel. The solution for the color balancing of a transparent segment differs from that of balancing the individual colors in that the color of the image created a transparent segment cannot be controlled by any form of temporal adjustment such as modifying the time of modulation or the relative size of the transparent segment. Rather, the color balance of that transparent segment will be determined by the spectrum of the illumination source and the relative spectral absorption characteristics of the remainder of the projection system between the lamp and the final projected image.
The technological area of Field Sequential Color (FSC) and FSC Projection is replete with terminology. Because the terminology has not become entirely standard within the industry the following definitions and clarifications are provided.
DATA FRAME (Input) is one set of input data delivered from the front end of the display system to the back end of the display system. A typical data frame rate is 60 frames per second.
COLOR FIELD (Output) is one set of data derived from the Data Frame for delivery by the back end of the display system to the display panel to enable the display panel to generate images for that one color. In a typical field sequential color system at least one color field is delivered to the display panel for each primary color included in the system design. In many systems additional color fields are delivered to the panel for each data frame. In a typical system the color field rate may be 360 fields per second.